Decoherence from classically undetectable sources: A standard quantum limit for diffusion

TL;DR

This paper establishes a fundamental quantum limit on detecting subtle diffusion effects caused by hypothetical particles or forces, suggesting matter interferometers can reveal phenomena otherwise hidden from classical detection.

Contribution

It introduces a standard quantum limit for diffusion that constrains entanglement effects from undetectable sources, highlighting the potential of decoherence-based experiments to explore new physics.

Findings

Decoherence can reveal classically undetectable phenomena.

Quantum limits restrict the influence of soft interactions.

Matter interferometers are promising tools for probing new particles.

Abstract

In the pursuit of speculative new particles, forces, and dimensions with vanishingly small influence on normal matter, understanding the ultimate physical limits of experimental sensitivity is essential. Here, I show that quantum decoherence offers a window into otherwise inaccessible realms. There is a standard quantum limit for diffusion that restricts some entanglement-generating phenomena, like soft collisions with new particle species, from having appreciable classical influence on normal matter. Such phenomena are classically undetectable but can be revealed by the anomalous decoherence they induce on non-classical superpositions with long-range coherence in phase space. This gives strong, novel motivation for the construction of matter interferometers and other experimental sources of large superpositions, which recently have seen rapid progress. Decoherence is always at least second order in the coupling strength, so such searches are best suited for soft interactions (e.g., small momentum transfers), but not weak ones (i.e., small coupling constants).